In optical communication technology, in order to increase the information density, an optical modulator based on a multi-level modulation scheme of 16-QAM or higher is under development.
As shown in PTL 1 or PTL 2, in a modulator having a configuration in which quadrature phase-shift keying (QPSK) structures are disposed in a nested structure, it is possible to generate an amplitude-shift keying (ASK) signal or a QAM signal by generating binary phase-shift keying (BPSK) signals, which are adjusted to have an appropriate light intensity ratio and an appropriate phase, and combining the binary phase-shift keying (BPSK) signals.
For example, in a structure shown in FIG. 10, when the amplitude ratio of the optical outputs from an optical modulation unit including a Mach-Zehnder type optical waveguide MZ1 and an optical modulation unit including a Mach-Zehnder type optical waveguide MZ2 is 2:1 and the phase difference is 0 or π, it is possible to generate the four-level ASK signal. In addition, when the amplitude ratio of the optical outputs from the MZ1 and the MZ2 in the structure shown in FIG. 11 is 1:1 and the phase difference is ±π/2, it is possible to generate the QPSK signal.
Specifically, by providing a light intensity adjusting unit for the waveguides of one QPSK structure as shown in FIG. 1, the output amplitude ratio of the QPSK signals is set to 2:1. In addition, the phase difference between the output light components from Mach-Zehnder type optical waveguides (MZ optical waveguides) in each QPSK structure is set to ±π/2, and the phase difference between the output light components from each of the QPSK structures is set to 0 or π. In FIG. 1, the points displayed with the X-Y coordinates schematically show signal states obtained by the output light from each Mach-Zehnder type optical waveguide or a combining section of optical waveguides.
In order to generate 16-QAM signals, as shown in FIG. 1, at least four MZ optical waveguides are required, and it is necessary to combine their outputs. Accordingly, the element itself becomes large even in the basic structure.
In addition, since it is necessary to make the amplitude ratio of the optical outputs from the MZ optical waveguides constant, a light intensity adjusting unit is required. As the light intensity adjusting mechanism, additional elements, such as an attenuator, MZ optical waveguide for intensity modulation, are used. However, these cause an increase in the size of the entire element.
As one of the light intensity adjusting unit, means for making the branching ratio in a branching portion of the optical waveguide asymmetrical may be considered. However, there are disadvantages in that the means is easily influenced by manufacturing deviations and the like. In addition, it is also possible to consider means for adjusting the light intensity by adjusting the amplitude ratio of input RF signals (modulation signals) in an external circuit. In this case, the number of adjustment sections for RF signal control is increased.
On the other hand, in the bias adjustment or the like of the BPSK signal in a conventional example, as shown in PTL 3 or FIG. 5, the bias point is a bottom of the modulation curve, the input amplitude is set to 2Vπ (Vπ is a half-wave voltage of the modulation curve), and a low-frequency dither signal with a frequency f is superimposed on the signal input. In the bias adjustment, for the optical output, a point at which the component with a frequency 2f is greatest is set as the bias point. In addition, the branched light of the optical output is used for detection, separated for each frequency using a band pass filter or the like. Each separated optical output is detected by a photodetection device, such as a photodetector (PD).
Since a plurality of optical modulation units, each of which is formed by one MZ type optical waveguide, are present, the two following methods may be considered in the selection of the dither signal applied to each optical modulation unit.
(1) Dither signals with the same frequency are applied to respective optical modulation units in a time-sharing manner.
(2) Dither signals with different frequencies are simultaneously applied to respective optical modulation units.
Since there need few frequencies used in the above (1), it is possible to set a narrow detection band. However, since the bias voltages of the optical modulation units are adjusted one by one, the bias points of other optical modulation units are drifted for the period. As a result, resistance to the bias drift becomes weak.
In the above (2), since a plurality of optical modulation units can be simultaneously adjusted, resistance to the bias drift becomes strong. However, it is necessary to detect a plurality of frequencies. For the control of the QAM signal, it is thought that the measures against drift are important. In general, therefore, the method in the above (2) with less time loss is preferable.
In the case of the above (2), however, a problem of the selection of dither signal frequencies occurs. That is, when selecting different frequencies of dither signals applied to the respective optical modulation units, the frequencies should be selected such that any frequency does not become twice the other frequencies. For example, assuming that the dither signal frequency used in a certain optical modulation unit is f, it is necessary to select frequencies other than the frequency of 2f or f/2 in other optical modulation units. Due to such frequency limitations, frequency selection of the dither signal is difficult in the QAM modulator with a large number of optical modulation units that are controlled.
In addition, as one of the methods to solve the above-described problem of the selection of dither signal frequencies, a method using a plurality of photodetectors (PD) for bias adjustment may be considered as shown in FIG. 6 or 7. In FIG. 6, a photodetector (PD) is disposed for each QPSK structure in which two optical modulation units having MZ type optical waveguides are disposed in parallel. FIG. 7 is a configuration in which one photodetector (PD) is attached to each optical modulation unit having one MZ type optical waveguide. In particular, in the case of the configuration shown in FIG. 7, it is also possible to use dither signals with the same frequency in bias control of a plurality of optical modulation units. However, space for attaching the photodetector (PD) is required, and electric wirings for routing the current output of light detection elements are complicated since the photodetectors are disposed away from each other.